Metal-oxide-semiconductor (MOS) transistors are the primary building blocks for modem integrated circuits. A typical highly integrated circuit, such as a microelectronic device, can contain millions of transistors on a single silicon substrate no bigger than a thumbnail. Generally, a transistor, or device and hereinafter referred to interchangeably, includes a gate structure formed on a substrate with a source region and a drain region, separated from each other by the gate structure and formed within the substrate, adjacent to the gate structure. A transistor may be thought of as an electronic switch having three nodes. When a voltage is applied to a first node of the transistor, i.e., the gate, the flow of electric current between the other two nodes, i.e., the source and the drain regions, via a channel region below the gate, is modulated. For example, to turn one type of n-channel (NMOS) transistor “ON,” a positive voltage is applied to the gate, allowing electric current to flow between the source and drain. To turn this transistor “OFF,” zero volts is applied to the gate which cuts off the flow of electric current between the source and drain.
The type of transistor on a microelectronic device varies depending on its intended function. Examples of transistors include NMOS and PMOS transistors used in Logic circuits and NMOS and PMOS transistors used in SRAM circuits. Generally, the function of Memory transistors require less power (and therefore slower current flow) while Logic transistors require more power (and therefore faster current flow). Power (represented by the formula Power equals I×V, wherein I equals current and V equals voltage) is measured by the speed of electrons moving from the source and drain regions via the channel region. One method of controlling this movement, and hence the power of a given transistor, is to control the distance from the source region to the drain region. Typically, because Memory transistors require less power, the distance from the source region to the drain region is larger when compared to that of a Logic transistor.
Distance between the source region and the drain region also affects leakage of current flow in the OFF state. “Leakage” is the amount of current flowing through the transistor when in the OFF state. Although a given transistor is in the OFF state, a small amount of current continues to flow through the channel region. The total current of a transistor is measured by the current flow in both the ON and OFF states. That is, current (I) equals ION+IOFF, where IOFF is very small compared to ION. The greater the distance between the source region and the drain region, the smaller the leakage. However, the trade-off is that the overall speed of the transistor is lessened.